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Measuring groups

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Measuring groups

Old 09-29-2004, 09:48 AM
Typical Buck
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Join Date: Feb 2003
Location: Rocky Mtn. Hse. Alberta
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Default Measuring groups

I was at the range yesterday and shot a Rem. model 788 in .243 that my kids use for pronghorn and deer. I was trying out some Factory Federal premium 100 gr. noslers. The gun hadn't been fired in a year and the first shot was a little low and right of where I expected. The next three were a cloverleaf about 3" high of the target center. I measured the group between the farthest edges(as I usually do) and it was 1/2 in. Then it struck me that I have read some people measure a group center to center of the bullet holes which are farthest apart. That makes this 100 meter group 5/16 in.

How do you usually measure your groups?

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Old 09-29-2004, 10:04 AM
Join Date: Aug 2004
Posts: 93
Default RE: Measuring groups

The most common is c-t-c. If you measure outside to you are penalizing yourself when you use larger ammo.

Think about it if you shot .25 inch group c-t-c in in .22 LR your group would be .47 to the outside...shoot that same group with a .50 cal BMG...your group would be .75 to the outside.

As well look at how national matches and olypic shooting measure...all c-t-c....
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Old 09-29-2004, 11:53 AM
Nontypical Buck
Join Date: Feb 2003
Location: Gypsum KS USA
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Default RE: Measuring groups

There are a great number of ways to measure a group, not all of them are right, but there are a GREAT number of "right ways" to do it.

Personally, I think you want to know the distance between the centers of the holes, since inside to inside or outside to outside would be affected by bullet diameter, i.e. if I shoot a 5" group center to center with a .22lr vs. a 5" C-C group with a .45-70, my outside to outside or inside to inside measurements will be a quarter inch off, when the actual center of mass of the bullet hit at the exact same spot.

How I do it: For 3shot groups, construct a triangle from center to center of each hole, then measure from the outside of one to the inside of another (gives you the length from center to center), then divide by two, Mark the middle of the line, and draw a line from the middle of your triangle sides to the center of the hole opposite that side. Do that for all three sides and you'll have the "center of mass" for all of your shots, basically the geometric center of your group.

Measure all lengths from the center of the triangle to the center of each hole, then average them. That is what I call my group size, and my longest length is my "widest shot".

It's easier to show you how this is done than to tell you, but I think that's clear, and it's a very accurate way of doing it. It takes a lot of time, so unless you really need to know the EXACT size of your group, this is kind of arduous.
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Old 09-29-2004, 12:53 PM
Fork Horn
Join Date: Dec 2003
Posts: 264
Default RE: Measuring groups

I measure to the outside of each of the two holes farthest apart and I subtract the bullet diameter. This gives you a pretty accurate center to center measurement. Not sure if this is the best way, but it's the way I do it.
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Old 09-29-2004, 04:27 PM
Giant Nontypical
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Location: Port Neches, Texas
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Default RE: Measuring groups

Center to center of the fartherest holes, or inside to outside, which gives you the same c-t-c spread. Its just easier to measure the outsides of the hole as compared to estimating the centers!
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Old 09-29-2004, 04:48 PM
Join Date: May 2004
Location: Hampstead, Maryland
Posts: 179
Default RE: Measuring groups

I take a fine ball point and draw a line through the centers of the holes and then measure across the widest point of whatever polygon is formed.
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Old 09-29-2004, 07:42 PM
Nontypical Buck
Join Date: Feb 2003
Location: Gypsum KS USA
Posts: 1,289
Default RE: Measuring groups

My problem with measuring from C 2 C of the widest shots is this:

Say I shoot a 3shot group and they're all three equidistant from eachother, i.e. an equilateral triangle, all three sides and angles the same...measuring from one side to the other, say all sides are 3" C 2 C. That's a 3" group if I measure it C 2 C, but in actuality, it's a 3.46" group...each shot is 1.73" from the center of the group, so if you shot 100shots that fit under the same circle as those three, you'd get a circle that was 3.46" in diameter, not a 3" circle.

Basically, where your bullet can strike is similar to a shotgun group, it's a conical path, so if your rifle is accurate to 1MOA, if you locked it in and shot hundreds of rounds with NO fliers, you'd get a circle at 1MOA, where 2 or three given points can be closer than that and still be ON THE CIRCLE, not just inside of it.

Measuring the two worst shots C 2 C assumes that one is exactly in the center and the other is the farthest from the center, so the other one is a "better shot" and can be disregarded. That's why the "center mass" method is what I use. It also helps you pin down how far EXACTLY you have to adjust your scope to center your group around the target.

Really to get more exact than the center mass 3 shot method, you've got to shoot more rounds, and even then the center mass method will give you a better representation of your accuracy.

Of course, most guys can't shoot consistently enough in their own form to make EVERY group the same, even in a vise, so the C 2 C method works, but if you only measure between the two holes farthest apart, why do you shoot three? or Five? Or ten?
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Old 09-29-2004, 07:44 PM
Nontypical Buck
Join Date: Feb 2003
Location: Gypsum KS USA
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Default RE: Measuring groups

By the by, it's usually easiest to measure your groups with a caliper (digital prefered), if you reload, you've already got one, if you're much of a "tool man", you've got one. it's much easier to "bracket" with a caliber than measure with a ruler.
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